1. Field of the Invention
This invention relates to an apparatus for reproducing a digital signal (digital information) from a recording medium such as an optical disk, a hard disk, or a magnetic tape. Also, this invention relates to a phase lock control apparatus usable in a digital signal reproducing apparatus.
2. Description of the Related Art
In a typical digital signal reproducing apparatus, a digital signal reproduced from a recording medium is periodically sampled at a timing determined by a sampling clock signal, and original data are recovered from the resultant samples of the reproduced digital signal. For accurate recovery of the original data, it is necessary that the sampling clock signal is correctly locked in phase with the reproduced digital signal.
In general, a digital signal reproduced from a recording medium is considered to be one of transmitted digital signals. In some cases, high-frequency signal components are attenuated when a signal travels along a transmission line. The waveform of a reproduced signal representative of digital information is smoothed as a result of attenuation of high-frequency signal components. Fluctuation signal components such as jitter signal components are sometimes caused when a signal passes through portions of a recording and reproducing apparatus. Noise adversely affects the level of a reproduced signal. These factors cause a sampling clock signal and a reproduced digital signal to be out of phase with each other. In other words, the factors cause a phase error between the sampling clock signal and the reproduced digital signal. The phase error decreases the accuracy of recovered original data.
Japanese patent application publication number 10-172250 discloses a digital signal reproducing apparatus including a phase lock control circuit. In the apparatus of Japanese application 10-172250, the phase lock control circuit has a provisional determination device for provisionally determining a likely value with respect to a digital information signal fed thereto.
The apparatus of Japanese application 10-172250 includes an A/D converter, a controller, and an error calculator in addition to the provisional determination device. The A/D converter changes a reproduced analog signal representative of reproduced digital data into a digital signal in response to a sampling clock signal. The provisional determination device provisionally determines the likeliest value with respect to the digital signal generated by the A/D converter through maximum-likelihood detection using a correlation between signal components. The error calculator generates an error signal indicative of an amplitude error on the basis of the result of the provisional determination by the provisional determination device. The controller adjusts the frequency of the sampling clock signal in response to the error signal generated by the error calculator. The sampling clock signal is fed to the A/D converter.
In the apparatus of Japanese application 10-172250, the sampling clock signal fed to the A/D converter is generated by an analog VCO (voltage-controlled oscillator). The analog VCO occupies a great space. Therefore, the apparatus of Japanese application 10-172250 tends to be large in size.
It is a first object of this invention to provide a small-sized information reproducing apparatus.
It is a second object of this invention to provide a small-sized phase lock control apparatus.
A first aspect of this invention provides an information reproducing apparatus comprising first means for periodically generating a sample of an information signal in response to a sampling clock signal; second means for detecting a level relation between successive ones of the information-signal samples generated by the first means; third means for detecting a peak-corresponding sample among the information-signal samples in response to the level relation detected by the second means and in response to a prescribed level range containing a xe2x80x9c0xe2x80x9d level, wherein a level represented by the peak-corresponding sample detected by the third means is outside the prescribed level range; fourth means for calculating a level difference between information-signal samples immediately preceding and immediately following the peak-corresponding sample detected by the third means; fifth means for generating a phase error signal in response to the level difference calculated by the fourth means; and sixth means for controlling the sampling clock signal in response to the phase error signal to lock the sampling clock signal in phase with the information signal.
A second aspect of this invention provides an information reproducing apparatus comprising an analog-to-digital converter for converting an analog signal representative of digital information into a first digital information signal; and a re-sampling DPLL section for periodically re-sampling the first digital information signal in response to a bit clock signal to convert the first digital information signal into a second digital information signal. The re-sampling DPLL section comprises (1) an interpolator for generating samples of the second digital information signal from samples of the first digital information signal through interpolation responsive to the bit clock signal; (2) a provisional determination device for detecting a peak-corresponding sample among the samples of the second digital information signal in response to a prescribed level range containing a xe2x80x9c0xe2x80x9d level, wherein a level represented by the detected peak-corresponding sample is outside the prescribed level range; (3) a calculator for calculating a level difference between samples immediately preceding and immediately following the detected peak-corresponding sample; (4) means for generating a phase error signal in response to the calculated level difference; and (5) means for controlling the bit clock signal in response to the generated phase error signal to lock the bit clock signal in phase with the first digital information signal.
A third aspect of this invention provides a phase lock control apparatus apparatus comprising an analog-to-digital converter for converting an analog signal representative of digital information into a sequence of samples of a digital information signal in response to a sampling clock signal; a provisional determination device for detecting a peak-corresponding sample among the samples of the digital information signal in response to a prescribed level range containing a xe2x80x9c0xe2x80x9d level, wherein a level represented by the detected peak-corresponding sample is outside the prescribed level range; a calculator for calculating a level difference between samples immediately preceding and immediately following the peak-corresponding sample detected by the provisional determination device; first means for generating a phase error signal in response to the level difference calculated by the calculator; and second means for controlling the sampling clock signal in response to the phase error signal generated by the first means to lock the sampling clock signal in phase with the analog signal.
A fourth aspect of this invention is based on the second aspect thereof, and provides an information reproducing apparatus wherein the provisional determination device comprises a provisional determination circuit for detecting a likeliest peak-corresponding sample among the samples of the second digital information signal on a maximum-likelihood detection basis.
A fifth aspect of this invention is based on the fourth aspect thereof, and provides an information reproducing apparatus wherein the provisional determination circuit comprises means for detecting the likeliest peak-corresponding sample in response to a threshold value, and means for varying the threshold value.
A sixth aspect of this invention is based on the fourth aspect thereof, and provides an information reproducing apparatus wherein the calculator comprises means for calculating a level difference between samples immediately preceding and immediately following a sample of interest, and means for holding the calculated level difference until the provisional determination circuit judges the sample of interest to be the likeliest peak-corresponding sample.
A seventh aspect of this invention is based on the fifth aspect thereof, and provides an information reproducing apparatus wherein the calculator comprises means for calculating a level difference between samples immediately preceding and immediately following a sample of interest, and means for holding the calculated level difference until the provisional determination circuit judges the sample of interest to be the likeliest peak-corresponding sample.
An eighth aspect of this invention is based on the fourth aspect thereof, and provides an information reproducing apparatus wherein the provisional determination circuit comprises means for detecting the likeliest peak-corresponding sample in response to a threshold value, means for detecting at least one of (1) a frequency condition and (2) an amplitude condition represented by the samples of the second digital information signal, and means for varying the threshold value in response to the detected at least one of the frequency condition and the amplitude condition.
A ninth aspect of this invention is based on the fourth aspect thereof, and provides an information reproducing apparatus wherein the analog signal representative of the digital information comprises a signal transmitted by a partial-response system using an inter-symbol interference.
A tenth aspect of this invention is based on the first aspect thereof, and provides an information reproducing apparatus further comprising a waveform equalizer for subjecting the information signal to PR (a, b, xe2x88x92b, xe2x88x92a) waveform equalization to generate an equalization-result signal, and a decoder for decoding the equalization-result signal into original information.
An eleventh aspect of this invention is based on the first aspect thereof, and provides a reproducing apparatus wherein the analog signal representative of the digital information comprises a signal reproduced from an optical-disc recording medium in a tangential push-pull method.